The present invention relates to an additive for fuel oils, containing ethylene/vinyl ester terpolymers and amphiphilic, lubrication-improving additives, and to its use for improving cold-flow and lubricating properties of the oils containing said additives.
Mineral oils and mineral oil distillates which are used as fuel oils generally contain 0.5% by weight or more of sulfur, which causes the formation of sulfur dioxide on combustion. To reduce the environmental pollutions resulting therefrom, the sulfur content of fuel oils is always further reduced. The introduction of the standard EN 590 relating to diesel fuels currently prescribes a maximum sulfur content of 500 ppm in Germany. In Scandinavia, fuel oils containing less than 50 ppm and, in exceptional cases, less than 10 ppm of sulfur are already in use. As a rule, these fuel oils are prepared by a procedure in which the fractions obtained from the mineral oil by distillation are refined with hydrogenation. During the desulfurization, however, other substances which impart a natural lubricating effect to the fuel oils are also removed. These substances include, inter alia, polyaromatic and polar compounds.
However, it has now been found that friction- and wear-reducing properties of fuel oils deteriorate with increasing degree of desulfurization. Often, these properties are so poor that signs of corrosion have to be expected after only a short time on the materials lubricated by the fuel, such as, for example, the distributor injection pumps of diesel engines. The further reduction of the 95% distillation point to below 370° C., in some cases to below 350° C. or below 330° C., which has now been implemented in Scandinavia makes these problems more critical.
The prior art therefore describes approaches which are intended to solve this problem (so-called lubricity additives).
EP-A-0 764 198 discloses additives which improve the lubricating effect of fuel oils and which contain polar nitrogen compounds based on alkylamines or alkylammonium salts having alkyl radicals of 8 to 40 carbon atoms.
EP-A-0 743 974 discloses the use of mixtures of lubricity additives (esters of polyhydric alcohols and carboxylic acids having 10 to 25 carbon atoms or dicarboxylic acids) and flow improvers comprising ethylene/unsaturated ester copolymers for the synergistic improvement of the lubricating effect of highly desulfurized oils.
EP-A-0 807 676 discloses the use of a mixture of a carboxamide and a cold-flow improver and/or an ashless dispersant for improving the cold flow properties of low-sulfur fuel oil.
EP-A-0 680 506 discloses the use of esters of monobasic or polybasic carboxylic acids with monohydric or polyhydric alcohols as lubricity additives for fuel oils.
The use of cold flow improvers in fuel oils is required since crude oils and middle distillates, such as gas oil, diesel oil or heating oil, obtained by distillation of crude oils contain amounts of long-chain paraffins (waxes) which differ depending on the origin of the crude oils. At low temperatures, these paraffins are precipitated as lamellar crystals, in some cases with inclusion of oil. This considerably impairs the flowability of the crude oils and the distillates obtained from them. Solid deposits occur and frequently lead to problems in production, transport and use of the mineral oils and mineral oil products. Thus, blockages of the filters occur at low ambient temperatures, for example in the cold season, inter alia in diesel engines and furnaces, and prevent safe metering of the fuel and finally result in an interruption of the supply of fuel or heating composition. Furthermore, the transport of the mineral oils and the mineral oil products through pipelines over relatively long distances may be adversely affected by the precipitation of paraffin crystals, for example in winter. It is known that undesired crystal growth can be suppressed by suitable additives and any increase in the viscosity of the oils can thus be counteracted. Such additives, which are known by the term pour point depressants or flow improvers, change the size and shape of the wax crystals and thus counteract an increase in the viscosity of the oils.
EP-A-0 807 642 discloses cold flow improvers based on terpolymers which contain structural units of ethylene, vinyl acetate and 4-methyl-1-pentene, and EP-A-807 643 discloses those based on ethylene, vinyl acetate and norbornene.
It has been found that, in low-sulfur and paraffin-rich oils, the synergistic combination of additives of the prior art, in particular in cold blending which is becoming increasingly important in practice, i.e. mixing additives into cold oils, lead to filtration problems above the cloud point of the oils containing said additives. The result is. often an impairment of the lubricating effect by the flow improver, and the oils do not have the properties expected of the components. For example, in the case of the additives according to EP-A-0 743 974, this is caused by the poor solubility of the flow improver component, with the result that blockage of fuel filters can occur. Presumably, the lubricants are absorbed by the more sparingly soluble components of the flow improver.